As an example of a conventional art, a patent document (Japanese patent No. 1999-20671A) discloses a control device adapted for a brake apparatus, in which at step S4 (see FIG. 6 of the patent document), whether a vacuum brake booster 12 is in a boosting limit state or not, is judged. If the control device judges that the booster 12 is in the boosting limit state, a pressure increase control is initiated (steps S8 to S13). In more detail, the control device actuates a pump 112 and generates a hydraulic pressure higher than a master cylinder pressure PM by a pressure difference ΔP and supplies the generated hydraulic pressure to a brake cylinder 50 during braking operation. Thus, the control device controls a pressure increase gradient (inclination) to be constant irrespectively of the booster 12 being during the time period before or after the boosting limit state. The pressure increase gradient indicates a gradient of a rate of change with time of a brake cylinder pressure PB. (Reference numerals and symbols described above are shown in FIG. 6 or FIG. 1 in the patent document, i.e., Japanese patent No. 1999-20671A).
In the conventional control device shown in the patent document is structured to judge whether the vacuum brake booster 12 is in the state of boosting limit based on a booster pressure signal from a booster pressure switch 304. This structure has a drawback that the pump 112 may be erroneously operated even when the vacuum booster 12 has actually not reached the boosting limit state due to a detection deviation of the booster pressure switch 304, or a mechanical deviation of the booster 12. Such erroneous operation may lead to insufficient exertion of a desired braking performance.
Need thus exists for a control device adapted for a brake apparatus which is not susceptible to the drawbacks mentioned above.